University of Illinois at Urbana-Champaign
Microanalysis Laboratory


Instruments and Techniques

All results for elemental analyses are calculated based on a known value of a standard. A computer then calculates the ratio of the sample output to the standard output to arrive at a value. The standards used are all traceable back to NIST primary standards and all instruments are checked with a NIST primary standards on a regular basis to assure day to day accuracy of results.

All results, unless noted otherwise, will be reported as % by weight.
 

CHN
Model CE 440 CHN Analyzer

Principles of Operation

The samples to be analyzed are weighed into consumable tin or aluminum capsules.

image of work spaceimage of sample

The capsule is injected into a high temperature (1000°C) furnace and combusted in pure oxygen under static conditions. At the end of the combustion period, a dynamic burst of oxygen is added to ensure total combustion of all inorganic and organic substances. If tin capsules are used for the sample container, an initial exothermic reaction occurs raising the temperature of combustion to over 1800°C.

CE-440 Flow Schematic Diagram

The resulting combustion products pass through specialized reagents to produce carbon dioxide (CO2), water (H2O) and Nitrogen (N2) and oxides of nitrogen. These reagents also remove other interferences including haloges, sulfur and phosphorus. The gases are then passed over copper to scrub excess oxygen and reduce oxides of nitrogen to elemental nitrogen. After scrubbing, the gases enter a mixing volume chamber to ensure a homogeneous mixture at constant temperature and pressure.

The mixture then passes through a series of high-precision thermal conductivity detectors, each conatining a pair of thermal conductivity cells

Between the first two cells is a water trap. The differential signal between the cells is proportional to the water concentration, which is a function of the amount of hydrogen in the original sample. Between the next two cells is a carbon dioxide trap for measuring carbon. Finally, nitrogen is measured against a helium reference.

ELEMENT RANGE DETECTION LIMIT ERROR
C 0-99% 0.10% 0.06%
H 0-85% 0.10% 0.06%
N 0-85% 0.10% 0.06%

Based on a 2.0 mg sample.

Source: Exeter Analytical, Inc.

Further reading

CE-440 Theory of Operation

CE-440 Specs

CHN Micro-Analysis: A Comparative Review of the Effects of Instrument Design on Analytical Performance